1. Field of the Invention
The present invention relates generally to a microwave energy for detecting a microwave transmitted from measurement equipment.
More specifically, the invention relates to the structure of a reception frequency converting portion of the microwave detector for frequency mixing a reception wave and an output of a local oscillator.
2. Description of the Related Art
Multi-band microwave detectors, such as so-called radar detectors which are adapted to detect microwaves transmitted from radar type speed measurement equipments for use in generating an alarm, have been known in the prior art. For example, U.S. Pat. No. 4,571,593, issued on Feb. 18, 1976 to Martinson, U.S. Pat. No. 4,939,521, issued on Jul. 3, 1990 to Burin, U.S. Pat. No. 4,961,074, issued on Oct. 2, 1990 to Martinson disclose radar detectors of the type that the present invention is directed. In the case of typical traffic monitoring radar type speed measuring equipment (hereinafter referred to as "radar") microwaves in a plurality of frequency bands are employed, i.e. 10 GHz band (X band), 24 GHz band (K band) and 35 GHz band (Ka band). The known radar detectors receive the microwaves transmitted from the radar through a heterodyne type receiver circuit, such as a super-heterodyne receiver circuit.
In such a type the microwave detector, a horn antenna is generally employed as a reception antenna. At a feeding point in a throat portion of the horn antenna, a mixing diode is provided. A microwave circuit comprising a local oscillator is provided inside of the throat portion of the horn antenna. The mixing diode performs frequency mixing of the reception signal received by the horn antenna and the output of the local oscillator. Also, in order to avoid the affect of the reception input through the horn antenna for operation of the local oscillator, an appropriate filter is provided between the mixing stage and the local oscillator. In the known art, there is a type having a construction in which the mixer and the local oscillator are formed by a microwave circuit employing a wave guide tube. Also, it is known to form the mixer and the local oscillator with a microwave IC employing a microstrip line.
For constructing a multi-band microwave detector covering all of the above-mentioned X band, K band and Ka band, it is not possible to cover all reception bands using a single local oscillator, and it is necessary to provide two local oscillators having mutually different frequency bands. In such a case, two local oscillators are selectively operated in a time sharing manner to switch the reception band. By such an arrangement, multi-band reception can be realized with the single horn antenna and the single mixer. One example of the microwave detector of this type has been disclosed in U.S. Pat. No. 4,952,936.
In the prior art disclosed in the above-mentioned U.S. Pat. No. 4,952,936, a microwave IC is formed with a mixer circuit and two local oscillators. The microwave IC employing the microstrip line is certainly beneficial with respect to the viewpoints of downsizing and making the circuit alignment free. However, a high precision in patterning of the strip conductor on a dielectric substrate is required, and thus high designing skill is necessary. This results in disadvantages in the viewpoint of machining precision requirements and production yield in production of the parts.
In this respect, the microwave circuit employing the wave guide tube permits easy adjustment of the characteristics by aligning means, such as an iris, pin or so forth, even after assembling the main body of the microwave circuit. Furthermore, know-how in the adjustment technology have been accumulated for many years. Accordingly, in view of the overall production, the microwave circuit is superior over the microwave IC. However, in the prior art, there is no multi-band microwave detector having a single horn antenna, single mixer and two local oscillators employing the microwave circuit.